The invention of Japanese Patent Application Laid Open [Kokai or Unexamined] Number 1-100281 [100,281/89] is an example of a surface treatment liquid for tin plated DI cans. This teaching of the prior art employs a film forming liquid for the treatment of metal surfaces. This solution has a pH of 2 to 6 and contains 1 to 50 grams per liter ("g/L") of phosphate, 0.2 to 20.0 g/L of oxyacid ions, 0.01 to 5.0 g/L of tin ions, and 0.01 to 5.0 g/L of condensed phosphate. Treatment with this conversion treatment solution afforded a highly corrosion resistant phosphate film on the surface of tin-plated DI cans.
However, in recent years tin-plated DI cans have been produced using low levels of tin plating in response to economic considerations, and this has required that its surface treatment provide far more corrosion resistance than before. Moreover, when treatment is conducted by prior methods, in some cases the gloss of the base metal is degraded due to etching of the base metal. Accordingly, there is a demand for a surface treatment which does not damage the external appearance by reducing the gloss.
Treatment methods intended to provide corrosion resistance and adhesivity through the use of water soluble resin are exemplified by the invention in Japanese Patent Application Laid Open Number 1-172406 [172,406/89]. This invention provided as an example of the prior art comprises a method in which the metal surface is treated with a solution which contains an effective derivative of a polyhydric phenol compound. However, the disclosed method does not generate a satisfactorily stable corrosion resistance.
In addition, the metal can manufacturing process often suffers from a problem with transfer or transport: the slideability of the outer surface of the can during conveyor transport of the can may be poor due to a high friction coefficient of the outer surface, so that the can may be tipped over sideways. Can transport to the printer in the most modern high speed can lines is a particular problem in this regard. Accordingly, there is demand in the can manufacturing industry for a reduction in the static friction coefficient of the outer surface of cans, which at the same time does not cause any adverse effects on the adhesion of any paint or lacquer subsequently coated on the can. The invention of Japanese Patent Application Laid Open Number 64-85292 [85,292/89] comprises a method for improving this slideability. The reference teaches a surface treatment composition for metal cans which contains water-soluble organic material selected from phosphate esters, alcohols, monobasic, and polybasic fatty acids, fatty acid derivatives, and mixtures of the foregoing. While the disclosed method does in fact generate an increase in the slideability, it does not improve the corrosion resistance or paint adhesion.
U.S. Pat. No. 4,517,028 teaches in general terms treatment of metals with aminated derivatives of poly(vinyl phenols). This reference, however, makes no specific reference to treating tin plate or DI cans.